The present invention relates to improved anaerobic fermentation processes for the digestion of solid waste slurries conducted at high solids concentrations and to improved fermentation reactors for use in said processes comprising mixing means.
Anaerobic digestion of municipal solid waste (MSW) couples the potential of producing considerable energy (methane) with the simultaneous reduction in organic-waste disposal problems by a much less energy-intensive process than conventional aerobic processing methods. However, several key issues must still be overcome before the methane produced is economically competitive with conventional sources of natural gas. Because the value of methane produced is relatively low, the anaerobic process must be rather simple in design, require little energy to operate, and maintain high gas production rates.
Economic evaluations of anaerobic digestion for production of gas from MSW, B. Goodman et al. Anaerobic Digestion for Energy Production, SERI/SP-231-2624 (1985), show that receptor capital costs are a significant economic burden because of the large reactor volumes required in current anaerobic processes operated normally at lower solid levels (3%-5%). However, the inventors have determined that if solids levels could be increased significantly (above 10%), gas production rates per reactor volume would increase as well, thereby lowering reactor costs.
Solid biomasses absorb water so readily that a slurry of biomass in water from about 10% to 15% solids concentration and higher more closely resembles a damp solid than a pumpable slurry. Thus, the viscous slurry that results cannot be mixed in conventional fermentation equipment, and water is typically added to lower the viscosity sufficiently (to 10% solid levels or less) that mechanical agitation is possible. Alternatively, the system may be operated without mixing, but inhomogeneities develop that result in lower conversion rates, thereby negating much, if not all, of the gain of high solids operation.
Historically, research on higher solids anaerobic fermentation has focused on the single charge (batch), non-mixed reactor concept, generally with recirculation of effluent S. Ghosh Solid-Phase Methane Fermentation of Solid Wastes, Eleventh American Society of Mechanical Engineers: National Waste Processing Conference, Orlando, Fla. (1984); R. P. Goebel, High Solids Anaerobic Digester for Rural Use, U.S. DOE Technical Report No. DE-FG03-81 SF-11613 (1983); Y. M. Lin Coupled High Solids Fermentation and Anaerobic Filtration of Cellulosic Residues, Ph.D. Thesis, Michigan State University (1983); Snell Environmental Group (SEG), Rapid Methane Generation from Solid Waste, U.S. DOE Technical Report No. DE-FG02-81R510329 (1983); R. LeGrand et al., Continuous Anaerobic Digestion of High Solids Biomass: Modeling and Experiments, U.S. DOE Project No. DE-AC-02-76ET20051, Vol. II (1980); S. J. Hall et al. Mesophilic Anaerobic Digestion of High Solids Cattle Waste in a Packed Bed Digester, J. Agric. Eng. Res. 32:153-162 (1985); W. J. Wujcik and W. J. Jewell Biotechnology and Bioengineering Symposium, No. 10, pp. 43-65 (1980).
With non-mixed systems, rates of gas production are generally much slower than in liquid, mixed system, M. Z. Lowenstein SERI Anaerobic Digestion Program, 1984 Annual Report, SERI/PR-231-2691; Snell Environmental Group, supra; W. J. Wujcik and W. J. Jewell, supra. The retention time of solids required to effect a near complete digestion of the substrate in non-mixed liquid systems is on the order of months for mesophilic temperatures and of weeks for thermophilic operation.
Other workers have investigated two approaches to mixed higher solids fermentation. T. Goldberg et al. An Anaerobic Digester for Continual High Solid Loading: The Bio-Funnel, International Gas Research Conference, Aarhus County, Denmark (1981), consisted of a funnel-shaped digester wherein materials loaded at about 21% solids were reduced to about 13% solids after passing upward through the bottom of the funnel and spilling over the top. Agitation of the material occurred as the material expanded in going toward the top of the funnel. Lowenstein supra attained solids feeding as high as 10% solids within the digester in a modified continuous stirred tank reactor (CSTR).
Applicants have surmised that if even higher solids concentrations could be efficiently processed, the reactor volume could be reduced significantly, and the economics of anaerobic digestion of MSW to produce methane, and the fermentation of other organic substrates, such as biomass to produce ethanol, might improve.
Available kinetic data suggest that production rates should increase with solids concentration in the reactor, V. J. Srivastava et al., Aquatic Plants for Water Treatment and Resource Recovery, (K. R. Reedy et al., eds.) Part 4:713 (1987). Applicants therefore hypothesized that if these data were substantiated, a decreased reactor volume would be possible for higher solids concentrations while maintaining the same solids loading rate and retention time. The higher conversion rates achievable at higher concentrations of solids will decrease the reactor volume even more.
A reactor design allowing high solids material to be well mixed would increase the interaction of substrate, microorganisms, and metabolic intermediates. Prior reactors for mixing and operation at solids level of 9% or greater have resulted in problems with material movement caused by substrate bridging, scum formation and zones of non-mixing within the reactors when the solids levels fall above 10%.
It has thus been shown that prior to this invention a substantial need for an economical apparatus for digestion of solid wastes and biomass existed, and that the fermentation industry had not been able to produce such an apparatus, nor to conduct high solids digestion in a reliably efficient manner.
The fermentation reactor of this invention is a significantly different concept that those used in high solids fermentation research by other investigators (Lowenstein, supra, and Goldberg et al., supra). This reactor, used in batch or continuous operation, provides adequate mixing of high solids materials having concentrations in the reactor of about 10% to 100%, i.e., damp solids rather than pumpable slurries.
Devices similar to the reactor of this invention have not been found to be available to the fermentation industry, although a search through the patent literature discloses several devices that have certain similarities to this reactor in non-analogous arts. U.S. Pat. No. Des. 8,004 to Reynolds for a Feather Renovator, discloses a hand-operated cylindrical container having a shaft equipped with non-symmetrically disposed rods, presumably for beating feathers. This patent does not disclose or suggest automatic rotation or any other special configurations useful for fermentation. U.S. Pat. No. Des. 286,293 to Germain for Design for Spreader Agitator, discloses a shaft equipped with short agitator members broader at the bottom than at the top, which appear to be designed for intensive, high sheer mixing that would destroy microorganisms or enzymes. The shortness of the agitator members with respect to the diameter of the shaft would cause inadequate mixing in a reactor large enough to have sufficient reaction volume, or in a smaller reactor, would provide insufficient reaction volume. U.S. Pat. No. 1,725,868 to Kenyon for Machine for Conditioining Gravel, shows a device that has too large a shaft diameter with respect to the vessel diameter to be useful in fermentation reactions, and that would require excessive power to operate. U.S. Pat. No. 3,012,977 to Wilson et al. for Method for Making Synthetic Resin Foams, depicts a device that has a vertical rather than a horizontal shaft with broad paddles. This device is designed for use with liquid chemical reaction systems, not with solids fermentation processes. U.S. Pat. No. 3,666,242 to Fifer for Waste Treatment Apparatus and Circulation Unit Therefor, discloses a stationary vertical circulation unit for aerobic digestion processes, with a lift impeller in the center. Japanese Patent Disclosure 60-150821 to Chisso Corp. for Horizontal single shaft cylindrical vessel stirring device--partic. for polymers such as polyethylene, polypropylene or polybutene, depicts a device designed with broad, flat paddles as the end of its mixing members, such that tacky polymer flows smoothly around the shaft and is stirred so as to wash around the shaft. None of these patents disclose a reaction vessel with mixing members suitable for fermentation of high solids materials.